The hypothesis underlying the proposed project is that a beta granule associated protein (betaGAP) is the principal antigen for a panel of islet-reactive, diabetogenic T cell clones derived from the NOD mouse. Our primary objective, and the first aim, of the project continuation is to purify this protein antigen to homogeneity in order to obtain a sequence and definitive identification. Under the second aim, we plan to investigate whether antibodies with reactivity to beta granules are directed to the same antigens as T cells. The third aim is to analyze T cell receptor (TCR) sequences from the T cell clones to determine whether any distinct patterns of TCR gene usage can be detected. The significance of the proposed work lies in the fact that while a growing number of candidate antigens is being investigated for relevance to diabetes, no one antigen has been identified as the principal target of the autoreactive T cells that mediate disease. Our panel of well- characterized, diabetogenic T cell clones provides one of the best systems available to investigate this question. The value of these T cells as tools to probe disease-relevant beta cell antigens is indicated by two important features: (1) their ability to rapidly induce diabetes in two-week old mice, and (2) all of the diabetogenic clones tested react to a beta granule membrane associated protein, suggesting an immunodominant antigen.